1. Field of the Invention
The present invention relates to a semiconductor laser and more particularly, relates to a semiconductor laser of a stripe structure having a buried layer of II-VI compound.
2. Description of the Prior Art
In recent years in the technical field of semiconductor lasers, a number of lasers of a stripe structure have been developed for the purpose of achieving a low threshold current and a single transverse mode oscillation. One of the objects in this development is to narrow the current path.
FIG. 1 shows a typical type of laser which comprises a substrate 1 of n type GaAs (Gallium Arsenide) the major surface of which includes a crystal face or facet, an oscillation layer 2 deposited on the substrate, a cap layer 6 deposited on the oscillation layer 2 and comprised of n type GaAs having a good ohmic characteristic, a diffusion region 7 of P.sup.+ type in which Zn (zinc) is partially diffused from the surface of the cap layer to a second clad layer 5, and first and second ohmic electrodes 8 and 9 which are formed on the surface of the cap layer 6 and other surface of the substrate 1. The oscillation layer 2 includes a first clad layer 3 comprised of n type Ga.sub.1-x Al.sub.x As(gallium aluminum arsenide)(0&lt;x&lt;1), an active layer 4 comprised of Ga.sub.1-y Al.sub.y As(0.ltoreq.y&lt;1, x&lt;y), and a second clad layer 5 comprised of P type Ga.sub.1-x Al.sub.x As, which layers 3, 4 and 5 are in turn epitaxially grown and deposited in that order.
In such a semiconductor laser, if and when a forward bias is applied between the first and second electrodes 8 and 9, a current path is narrowed within the diffusion region 7 and a current flows immediately beneath the approximate diffusion region 7. In addition, since the concentration of aluminum in the first and second clad layers 3 and 5 is larger than that of the active layer 4, the band gap of the clad layers 3 and 5 becomes larger and the index of refraction thereof becomes smaller, as compared with the active layer 4.
Accordingly, electrons and holes are well confined in the active layer 4 immediately beneath the diffusion region 7 and also the light caused by recombination of such electrons and holes is well confined in the active layer 4, whereby the laser beam in the single transverse mode with a low current is oscillated from the active layer 4.
On the other hand, in manufacturing the above described laser, the laser must be exposed in the high temperature of 600.degree. C..about.650.degree. C. in forming the diffusion region for narrowing the current path. If and when the laser is subjected to high temperature, thermal damage in a growing layer such as the active layer 4 and the like occurs and hence the life of the laser becomes shorter, which was a significant problem. In order to solve such problem, a structure was devised wherein, as shown in Fig. 2, a cap layer 11 is formed in a stripe geometry by P type GaAs and both sides of the cap layer are buried by a layer 12 of high resistivity which is comprised of SiO.sub.2 and the like. The laser as shown in FIG. 2 will be explained in the following. The same portions as FIG. 1 are denoted by the same reference numerals so that the explanation thereof is omitted.
In such laser, since the cap layer 11 is formed in a stripe geometry and is buried by the high resistivity layer 12 in both sides of the layer 11, the current path is narrowed to the cap layer 11 by the high resistivity layer 12. Accordingly, the laser beam in a single transverse mode with a low current can be oscillated just as the laser in FIG. 1.
In addition, since the above described high resistivity layer 12 is comprised of SiO.sub.2 and the like, the manufacturing process can be made at a relatively low temperature of approximately 300.degree. C. Thus, the problem of shortening the laser life due to thermal damage of the active layer 4 and the like never arise, as the laser of FIG. 1.
However, since thermal expansion coefficients of SiO.sub.2 and GaAlAs are different from (SiO.sub.2 : 0.35.times.10.sup.-6 /.degree.K, GaAlAs: 6.0.times.10.sup.-6 /.degree.K), crystal strain occurs in the second clad layer 5 and the like. The crystal strain mainly occurs in the laser manufacturing process and, more particularly, the strain occurs when the laser is cooled to a normal temperature after the high resistivity layer 12 of SiO.sub.2 is formed at the temperature of about 300.degree. C.
Furthermore, the surface of the semiconductor laser on the side of the cap layer 11 is fixed to a heat sink (not shown) for the purpose of enhancing heat radiation. However, the thermal conductivity of SiO.sub.2 is relatively low, that is, 0.014 W/cm..degree. C., the heat caused in the oscillation layer while using the laser can not be transferred to the heat sink and thus is stored within the laser, which causes a thermal break of the oscillation layer 2.
Typical examples of the above described prior art semiconductor lasers are disclosed in Japanese Laying Open Gazette Nos. 86093/1977, 18087/1980, 86181/1980, and 93381/1979. The semiconductor laser disclosed in the Japanese Patent Laying Open Gazzette No. 86093/1977 includes a buried layer comprised of SiO.sub.2. The semiconductor laser in Japanese Laying Open Gazette No. 18087/1980 includes a high resistivity layer (buried layer) which is formed at the temperature of 600.degree. C. by means of metal-organic chemical vapor deposition using Ga(CH.sub.3).sub.3 and AsH.sub.3. The technique disclosed in the Japanese Laying Open Gazette No. 86181/1980 is directed to an active layer in a semiconductor laser and does not disclose material for a buried layer. The semiconductor laser in Japanese Laying Open Gazette No. 93381/1979 includes a current blocking region (buried layer) comprised of n type Ga.sub.1-z Al.sub.z As(1&gt;z.gtoreq.0) which is formed after melt-back of a clad layer.